One of my videos renders properly and quickly while another not only takes ages, but just stops rendering half way and plays the stuff that has already been rendered
No offence, but you really haven't told us a thing. Please give a detailed explanation of the problem you're experiencing, the content of your projects, the results you expect, and the results you're getting etc.
well i have a video which i recorded. I click the play button and it lags very badly. so i click RAM preview and it doesnt fully render it, it goes to about half way and then just plays the vid. I dont udnerstnad why the whole video wont render.
After Effects doesn't work like a non linear editor. Content must be loaded into RAM to play back. Play back is limited to the amount of accessible RAM in your system.